Cellular communication systems are known. Such systems are typically made up of a number of substantially continuous service coverage areas (cells) within which a mobile communication unit may receive communication services through a centrally located base site of a nearby cell. Base sites, of the system, are typically under the control of a central controller which may interconnect with a public switch telephone network (PSTN). In addition to routing of calls between mobiles in different cells, or between a mobile and PSTN subscriber, the central controller also controls and facilitates handoffs of mobiles between adjacent cells. The Groupe Special Mobile (GSM) Pan-European digital cellular system, as specified in GSM recommendations available from the European Telecommunications Standards Institute (ETSI) and incorporated herein by reference, is an example of just such a system.
Communication services within a cellular system are typically provided within a frequency spectrum reserved for cellular services. Communicated signals are typically exchanged between a mobile and a base site using a radio frequency (RF) signal. The signal is typically exchanged over a frequency pair (transmit and receive) referred to, generally, as a communication resource. The frequency pair of the communication resource allow duplex communication from mobile to base on an inbound channel and from base to mobile on an outbound channel.
Information exchanged over a communication resource between mobile and base site may be exchanged under a number of communication protocols. Examples include frequency division multiple access (FDMA), time division multiple access (TDMA), or code division multiple access (CDMA). CDMA, in turn, may be divided into direct sequence spread spectrum (DSSS) or frequency hop.
CDMA systems allow for multiple, simultaneous users of the same spectrum through use of spreading codes that are identical among communicating transceivers but different among competing transceivers. In the case of slow frequency hopping communication systems, the spreading code is a unique sequence of communication resources over which a communication unit and base site hop, exchanging a signal on each communication resource for only a short period before hopping to the next resource. During the short period on each resource a frame of information is exchanged between base site and mobile before the communicating transceivers index, synchronously, to another resource in the sequence. Since each resource is used for only a short period and since each sequence may be different, a number of competing transceivers may coexist within the same spectrum.
The exchange of signals, at a base site, between communication units and base sites under frequency hopping may occur under a number of different methods. Under one method (referred to as baseband hopping) a number of transceivers, equal in number to the hopped frequencies, and a matrix switch are provided at a base site in support of frequency hopping. Having an equal number of transceivers as hopped frequencies allows the transceivers to be individually tuned to each of the hopped frequencies and for the transceivers to be selected individually, by the matrix switch, under the spreading code for the communication transaction. Upon selecting a transceiver the matrix switch transceives a communicated signal with the communication unit for a single frame before selecting another base site transceiver for exchange of another frame.
In other frequency hopping systems a single base site transceiver is assigned per communication transaction. The transceiver, in such a case, is programmed to index along with the communication unit through the sequence of frequencies allocated to the communication transaction.
While the use of a single base site transceiver per communication transaction works well in the simple case, problems may be experienced where a number of communicated signals are exchanged per frame under a time division multiplexed (TDM) format or where hopping occurs very rapidly. In either case less time per TDM slot or per frame may be available for the exchange of information. Synthesizer settling time of the base site transceiver, in such case, becomes a significant problem in the reliable exchange of information.
While providing a number of transceivers equal to hopped frequencies at a base site (as in baseband hopping) may be a solution to the synthesizer settling problem, considerable expense is involved. In lightly loaded systems, with few users, the baseband transceivers can remain unused much of the time. Because of the need for reliable communication networks a need exists for a more efficient method of operating base site transceivers in dynamically loaded frequency hopped communication systems.